phases: introduce phasecache
The original motivation was changectx.phase() had special logic to
correctly lookup in repo._phaserev, including invalidating it when
necessary. And at other places, repo._phaserev was accessed directly.
This led to the discovery that phases state including _phaseroots,
_phaserev and _dirtyphase was manipulated in localrepository.py,
phases.py, repair.py, etc. phasecache helps encapsulating that.
This patch replaces all phase state in localrepo with phasecache and
adjust related code except for advance/retractboundary() in phases.
These still access to phasecache internals directly. This will be
addressed in a followup.
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24